The present invention relates to apparatus for maintaining the mean values of the luminance and/or saturation of a video signal comprising a first path conveying a video signal with pre-emphasis having a pictorial content. The invention also relates to a method for maintaining such mean values.
In the HD-MAC television system currently being established in the EUREKA EU 95 high definition television project a problem has been found in the way in which gamma-corrected television signals are processed within an HD-MAC encoder which emphasises the boundaries between blocks which have been filtered with different characteristics. This problem has been reported in the paper "Colorimetry and constant luminance coding in a compatible HDMAC system" by S. C. Chen, F. Le Goff, R. Melwig and R. Schafer, IBC 88, Brighton, Sept. 24-27, 1988. When this effect is visible it overrides all other artifacts and can be particularly obtrusive since it occurs over a large area and can still be observed at viewing distances so great that the detail in the picture can no longer be resolved. The above paper proposes to use a true constant luminance system which would overcome these problems but, since a linear luminance signal must be digitized with at least 11 bits to avoid excessive noise near black, this would substantially increase the size and cost of the encoder and more significantly the decoder. An alternative unfiltered gamma-corrected representation may be digitized with 8 bits but this requires a higher sampling frequency. It seems unlikely that video tape recorders capable of storing this extra data will become economically available for some time.
It is an object of the invention to provide apparatus and a method that can be used to overcome the above problem and which can also be used in other circumstances.